In a small bowl, stir the milk with the granulated sugar and the yeast. Let stand for 5 minutes, until the yeast is foamy. Meanwhile, in a standing mixer fitted with the paddle, cream the 6 tablespoons of butter with the brown sugar, salt, nutmeg and orange zest at medium-high speed until light in color, about 2 minutes. Add the egg and vanilla and beat for 1 minute more. Add the pumpkin puree and beat until incorporated. Pour in the yeast mixture and beat at low speed to fully incorporate. The mixture will look curdled.